The present invention relates to an apparatus for producing a high-purity SiC single crystal by the continuous sublimation and condensation of SiC prepared by vapor-phase synthesis. The apparatus is especially suitable for manufacturing a long and large-sized single crystal.
SiC single crystals which have been produced by the sublimation process are useful as functional materials, e.g. semiconductor devices, in various industrial fields.
In a method belonging to the sublimation process, a powder SiC material is received in a graphite crucible having covers, a seed crystal is attached to the cover plate at a position facing to the powder SiC material, which is heated at a temperature of 2000.degree.-2500.degree. C. The heated material is evaporated from the crucible and condensed on the surface of the seed crystal. The condensed SiC grows to a single crystal having a crystalline orientation aligned to that of the seed crystal.
In another method, a seed crystal is attached to the bottom of a graphite crucible, a perforated graphite hollow cylinder is located in the crucible, a cavity between the inner surface of the crucible and the hollow cylinder is filled with a powder SiC material, and the powder material is heated at a high temperature. The heated powder material is evaporated, let permeate through the holes formed in the hollow cylinder, and condensed on the surface of the seed crystal at the bottom of the crucible.
The crucible to be used in these methods is made of high-purity graphite to inhibit the contamination of an obtained SiC crystal with inclusions. Even when graphite material of highest purity available on the market is used for the crucible, it contains impurities in an amount of approximately 5 ppm. The impurities are evaporated from the wall of the crucible during high-temperature heating, and mixed in a growing SiC crystal. The impurities included in the obtained single crystal causes various problems such as the deterioration of function and malfunction, when the single crystal is used as an active material such as a semiconductor device.
It is postulated that the impurities in the crucible material are introduced into the single crystal according to the mechanism of: When the powder SiC material is evaporated in the crucible, the sublimation product, i.e. SiC gas, does not always have a stoichiometric composition, but contains Si, Si.sub.2, C, SiC.sub.2, Si.sub.2 C, etc. in a mixed state. These components are reacted with each other to form SiC gas, and consumed for the growth of the SiC single crystal. In addition, SiC is also produced by the reaction of the gaseous components such as Si, Si.sub.2, SiC.sub.2 and Si.sub.2 C with C in the wall of the crucible. During the reaction with the crucible, the impurity elements in the crucible are evaporated at the same time and introduced into the growing single crystal.
The graphite crucible itself is consumed with the evaporation of C, so that the crucible changes its structure and the thickness of its wall. The changes in the structure and the wall thickness exhibits influences on the temperature gradient along the longitudinal direction of the crucible. Consequently, conditions for crystal growth fluctuate, so that an obtained single crystal has poor homogeneity and lower reliability in quality.
In the sublimation process using a graphite crucible, there are restrictions on the size of a usable crucible and the volume of a powder SiC material capable of being charged in the crucible. Due to the restrictions in a conventional method, it is practically impossible to produce a SiC single crystal having a diameter above approximately 30 mm or a length of several tens millimeter.
On the other hand, a single crystal larger in both diameter and length is required for enhancing productivity in the processing line of semiconductor devices. A SiC single crystal produced in the conventional process does not have a size sufficient to meet the requirement. In this regard, the production of SiC single crystals does not come onto the practically full-scale stage.